how can i pass the conv=ucase and conv=lcase in bash script?

how can i pass this dd command utility conv=ucase and conv=lcase in bash script?

if for example i have sample menu like:

Menu for translating contents of file

A.) translate contents of file to lowercase
B.) translate contents of file to uppercase

how to do this in the script so that the contents of file will become lower case if i choose letter A

... and

the contents of file will become uppercase if i choose B...??

by means of script only...